Comment from http://electronics.stackexchange.com/questions/356185/nand-flash-retention-using-usb-charger/356190?noredirect=1#comment855287_356190 claims that reading from flash storage is no lossless operation, so they get refreshed by the flash controller on spare time.
These filesystems do not use journaling, so ejecting the card or losing power during a write can corrupt it. Even with journaling, power loss during writes on flash is a touchy issue... you might lose more than the data that is being written, and it can spread to adjacent blocks too. A simple write may cause the controller to decide to move data around, etc... https://cseweb.ucsd.edu/~swanson/papers/DAC2011PowerCut.pdf
Doesn't repeated refreshing wear out these floating-gate transistors?